System for secret signaling



Jan. 3, 1928. r 1,654,900

L. J. SlVlAN SYSTEM FOR SECRET SIGNALING Filed April 23. 1925 s She ets-Sheet 1 /0 H9] 4 A? I? 4 1 r =l m- I mremom' leon J. Sir/an L. J. SIVIAN SYSTEM FOR SECRET SIGNALING Jan. 3, 1928. 1,654,900

Filed Aprii 23, 1925 s Sheets-Sheet 2 fiyZa IFZl-l/ IFZZ-O R250 I RUIZ Patented Jan. 3, 1928.

funrran stares- PATENT @FFHCE.

LEONJ. SIVIAJST, OF BLOOMFIELD, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGNMENTS, TOWESTERN ELECTRIC COMPANY, TIICORPOR-ATED, A CORPORATION OF NEW YORK.

SYSTEM FOR SECRET SIGNALING.

Application filed April 23, 1925. Serial No. 25,182.

This invention relates to systems for secret communication, and more particularly to'means for combining and separating noise frequency waves and signal frequency .waves.

Systems for obtaining secrecy in signaling have heretofore been devised, wherein the signals are obscured or rendered unintelligible, when received in the usual manner, by combining therewith noise waves.

Thepresent invention utilizes thesame general principle, in communication systems, to render secret the signals transmitted thereover, but applies this principle in a new and novel manner to effect results not heretofore obtained. The term signals, hereinus-ed, is intended to include speech, telegraph signals, music and the like or any portion of the frequency spectrum composing-such signals.

It is a general object ofthis' invention to secure privacy of communication.

, Another-object isto render messages unintelligible to persons unauthorized to receive them and to enable intelligible reception thereof by authorized persons.

A further object is to combine noise waves with signal waves to confuse the signals.

Another object is to remove the confusion from signal waves to enable the intelligible reception of signals. 7

An additional object is to transmit simultaneously with signal waves, noise waves of amplitude and frequency such as to cause masking of signals by noise in the ear of an unauthorized listener attempting to receive the signal waves.

A further object of the invention is to vary the volume of noise combined with signals in inverse proportion to the variations of volume of signab, whereby the total volume of noise and signal energy is maintainedalways within the limit of the maximumpeaks of signal power.

A further object is the transmission of noise or confusion waves or currents with signal waves or currents without increasing the power capacity of or overloading the transmitting, receiving, or repeating apparatus.

A feature of this invention is a means for combining noise waves with signal waves in predetermined amounts in accordance'with predetern iine d levels of signal energy.

Another feature is an arrangement for controlling the volume of noise in accordance with the volume of carrier waves modulated in accordance with signals. 7 V

A further feature relates to a system, whereby noise waves are combined with signal waves, the volume of noise waves being controlled in accordance with the volume of signals, whereby the volume of noise com-- bined with signals is varied continuously and inversely as the signal volume. A still further feature is a receiving system including electrically resonant elements for separating noise waves from signal waves to enable the intelligible reception of signals.

Another feature is a receiving system including mechanically resonant elements for discriminating between signal and noise fre quencies to effect the exclusion of noise waves from a signal receiver.

A further feature is an arrangement for increasing the selectivity of the resonantelements in the receiving system.

This invention operates upon the principle of, interference between signal waves and noise waves. It is a well known characteristic of'speechthat only a relatively small percentage of the total speech time is o ccupied by the maximum or near maximum amplitudes in the speech waves. It is, therefore, evident that during the greater per centage of the speech time additional energy may be combined with the speech energy without the total energy exceeding that represented by the peaks of speech waves. Secrecy of signals in a system for trans mitting speech is herein attained by introducing noise including components of suitable frequency and amplitude to obtain the greatest degree of masking of signals by noise. Tire noise is controlled as to the aniplitude and frequency of its components by the amplitude of th'e'speech. The frequency of the noise components is preferably related to the signal frequencies in such manner that advantage may be taken of the greater masking efficiency of low frequencies as compared to high frequencies.

The invention operates upon the further principle that signal waves may be masked or rendered unintelligible by applying to the ear of the listener at the proper instant, noise waves thatare of the proper frequency and volume to produce aftemporary overloading of the ear. In other words, by applying a sound wave of a particular frequency and of considerable volumeto the car oi the listener, immediately preceding the application thereto of undistorted signalwaves, advantage may-betaken of the fact that a finite time 1s=1 equ1rcd for'qthenection, With receiving systems. of Wellknowncharacter, for segregating the noise Waves from the signal Waves andexciud ng the forn'iertrom the rccei-vin devices. v r in the drawings, Fig. 1 illustrates an. arrangement for combiningnoise waves with signal waves, in fixed amounts,:the volume ofnoise being determined by'predetermined signal volume levels. I V i Fig.1 illustratesa modification of one ele Fig. 2 illustratesan arrangement wherein.

theyolumeof noise is controlledin accordance With the amplitude of modulated carrier Waves.

Fig. 2 is a graphiillustr ating the ment and operation: ofthe circuit illustrated in accordance, With the'volume of signals, is

illustrated. I

Fig. t; illustrates a signal-receiving system including -.e.lectrica11y resonant elenients fior segregating-no se andsignal Waves.

Fig. 5 illustrates a re'ce lar't'ov Fig.4.

ng system s m FigJ G illustrates a; receiving circuit, lion separating noise and signal Waves, including electrically resonant elements coupled to, a.

regenerative circuitfor obtaining increased selectivity; 1

Fig. 7 illustrates'a receiving circuit including mechanically, resonant elements, to, dis-,

.criminate betWeennoiseand signal Waves.

The circuit of Fig. lwill-now betdescribed.

This circuit includes terminals. 10 and 11. connected to the cathode and control electrode,

ofspace discharge devicef12,.which terminals are supplied Withsignal waves from any de.-,

sired "source as, for eXa-mple, a telephone .transmitter, -a telephone line, an antenna or the like. vThe lead,co nnectin'g terminal 11 to. the cathode of tube l2,.incl udesa polarizing battery 13'101' applying asuitable averagepOtential to the controlelectrode-of the tube. Anou'tput circuit for the device. 12is conn'ected toits anodeand cathode and includes the primary Winding. of transformer 14. in

seriesJvith space current battery 15. secondary .wm i 'ng Oftranstorm r c011 The I wherein the combined signal and noise Waves 7 circuit of a secon dfspace discharge'tube'20f electrode. An output circuit for tube i s."-

rent hattery2it. The cathode of tubes12 and :20 may belicated in any suitable nanneras;

' In Fig. 3am arrangement, for varying the.

7' volume of noise .C, Q y nve s y 'included'in a; circuit comprising, 'in series.

resistances 30, 31 ,and 32. reduce tofa neglt transmit-ted from generator l33 transformer 18 to the terminals 16 and 17 a'bly, of such voltageas to supply v p to thepolaiuzmg )VllltllDQSOf theassociat predetermined values may be difit'erent-for that, operation ofeachrelay. may takc plac'e' nected to output terminals 16 and 1 7 in series With the primary Winding of a transformer 18. The terminals 16,and- 17 are],- arranged to supply signal Wavescombined With/noise \ViLVBS to any desiredtransmit? l ting circuit such,1for example, as an ordinary telephone lin'eor a modulator, included n a radio orcarriercurrent transmittlng system, i.

are caused'to modulate a-high frequency carr er wave for transmission to a receiv ng system either over a wireli'ne or throughthe ether;

'A transformer'lfi) has its primary Winding included in the input circuit oftube 12 and ts secondary winding includedinthe input .he latter input circuit ishonhectedto the]. I cathode and control electrode ottube 20 .and includes alsofthebattery 9 efor-jmaintaining' a suitable average potential upon the control I conncctedtoits anode andcathode, andjin Cl llClGf-Eylll series, respective windlngs'ofpo larizedqifelay's 21,22, and 23 and space cur-.5

for example, by batteries. 25 and 26 connect- 1 23 include polarizing windingslconnected';i?e spectively to dnectcurrent'sources 27., 128,

1 The primarywinding of transformer l8 is resistances 30,31, 32, inultiple frequency; ge' er fiQ f 33., and; adjustable resistance 34. The

gible. amount, .t-he 'fvolume of noise energy I;

1 The resistances 30,31, and 32fareff c'on nected i espectively'fto :contacts andarina, 2 tures otreljays21, 22, and23' and are nor mally. short circuited thereby.

= l The batteries: 27", .28'and 29 are ed relay's in such direction that thei'inagnetic field-produced in the core of tlierelayl in each casetends to-opp'oso the field pro-. j

mature vcontact i thereofnormally Closed," for. v

currents inthe output circuitof tuhe go bef low certain. predetermined values; These each .relay,;as determined by the nmge of the i'espee"vef.batteries 27, 28,1 and-29,;so.

ata. different current. level inthe Loutput V v circuit to causetheopening of its associatedimd" iioniactsi' Upm'the i;e m- .qii-eae i zeia its associated resistance 3-0, 31 or 32 is effectively included in the circuit of generator The amplitude of noise waves, supplied from generator through transformer 18, to be combined with signal waves in the circuit connected to terminals 16 and 17, is thereby correspondingly reduced.

The level of current in the output circuit of tube 20 is determined by the potential applied to its control electrode by a source 9 and by signal waves supplied through transformer 19. It is clear, therefore, that the amplitude of the noise waves is controlled by variations in the an'iplitude of the signal waves.

In the operation of Fig. 1, signal waves supplied to terminals 10and 11 are transmitted to the input circuit of space discharge tube 12, wherein they are amplified and supplied through transformer 1 1 to the output terminals 16 and 17. Signal waves are also supplied through transformer 19 to the input electrodes'of tube 20 to control the space current flowing in the output circuit of this tube. When the negative potential applied to the control electrode of tube 20 by the battery 9 is reduced by the positive portions of the signal waves, the space current flowing in the winding of relay 21 may-be such as to cause the operation of this relay, but relays 22 and 23 may remain inoperative for this particular value of space current. In this event, the armature of relay 21 opens the contact associated therewith to short-circuit resistance 30. A decreased current, therefore, flows in the circuit including generaton33 and primary winding of transformer 18, whereby currents of noise frequencies of a lower value are produced in the output path including the secondary windings of transformers 14 and'18, where they are combined with the signal waves present in this path. As-the space current in tube 20 is still further increased by increasingsignal potentials applied to the input circuit ofthe tube, the relays 22 and 23 will operate, in the manner described in connection with relay 21, to include the respective resistances 31 and 32 in series with generator 33 and thereby reduce the noise currents flowing in the circuit in steps to a substantially negligible value. v

- In this manner, the volume of noise combined with the signals is determined by the volume level of the signals, and is varied inversely as the signal volume; also, since at the peaks of signal power the noise currents are reduced to a negligible amount, the total power transmitted, including both noise and signal power, never exceeds the peaks of signal power.

By virtue of this fact, the apparatus included in a signaling system embodying the present invention need not be designed to handle a greater amount of power than would be required without the application of noise currents. This advantage is of particularly great importance in signaling systems of high power, wherein the expense of providing the necessary facilities in creases rapidly with increase of power capacity.

The generator 33 may be designed to pro vide waves of any desired frequency, but it is preferably designed to generate waves of a plurality of frequencies, spaced within the audible range and including those frequencies which are found to be most effective in masking speech components. By doing this the number of components of different frequency necessary in the noise wave to produce complete masking of signals is materially reduced.

In a practical arrangement, this generator may be designed to produce waves having frequencies of 150 cycles, 600 cycles, 725 cycles, 1000 cycles, 1500 cycles, and 2100 cycles. These frequencies however, may differ in number and value without substantial change in'the effectiveness of the system.

In orderto further insure complete secrecy the different frequencies composing the noise may bevaried in the manner described in Patent No. 1, 161,T83, issued to It. D. Parker et 211., July 17, 1923.

In Fig. 1, a device which comprises a resonant circuit including an inductance 35 in shunt to a condenser 36, may be substituted for the element 32 included within the small dotted rectangle in Fig. 1. The inductance 35 and condenser 36 constitute an anti-resonant circuit, which may be tuned as desired to any one of the frequencies produced by generator 33, but is preferably resonant at a frequency of 1000 cycles per second. Let it be assumed that at a particular instant the volume of signals is such to cause the operation of relays 21, 22, and 23, a subsequent reduction in the signal volume below preassigne-d' values will then cause the successive release of these relays,

and the successive short-circuiting of their associated resistances 30,31, and 32 and accordingly the successive increase in volume of noise supplied to terminals 16 and 17. If the device of Fig. 1 is assumed to be substituted in the circuit of Fig. 1 in place of resistance 32, the noise waves supplied to terminals 10 and 17 would normally not in clu'de a component having the frequency of 1000 cycles, since this component would be impeded by the resonant circuit 35, 36. However, when relay 23 causes the resonant circuit to be short-circuited, waves of the frequency of 1000 cycles and of large vol ume, will be included among the noise frequencies. The volume of noise, supplied under these circumstances. together with the signal waves, may be such as to cause the listeners ear to be temporarily unable totinterpret E intelligible signals of :eonslderable volume supplied thereto immediately succeeding the application of the largevolume of-noise. It is. thus apparent that this in;

vention makes use otf the intervals when the signal volume is low to prevent a listener from hearing signals when the signal volume is high, and the noise volume is low; v350 V former let in serieswith a space current The secondarywi'nding oi trans source 15. former 1 1 may be included inthejnp'ut circuit of a space discharge tube-modulatorof any well-known type 'suc-h'tor example,as that describedin Patent .llo.v 1,350,752fto H; J; Van Der Bijl,-issucd August 24, 1920,

represented by block 37v in series with the Vprimary'winding of a transformer 18. The

modulator 37fmay have its output circuit connectedto the i'nput'circuitof a space discharge tube amplifier 38 which is-designed to efficiently translate and amplify high frequ eney Waves and may be in general similar toLthe amplifier v12. The output. circuitof ing of 1a" transformer w and'niaybe connected to output ternnnalslfi and-17 The secondary Winding'of transformer 19 isin- {eluded-iii the input circuit of a space discharge tube 80in series with a grid polar-V izing, battery 81. The output circuitof the,

I y-tube connected; to its "anode and cathode.

includes series, the primary winding of 'transforin'er82 and space current battery '83.

i The transformerwinding is shunted' a by-pass .con denser 18.4 .01" low impedance for.

high frequency waves, 1. e. carrierand side' band frequencies, An arrangement, reprehand pri1'nary windings of transformers 82 andl8'wasshown.g is WVhen-thfe arrangement in F g; '21s in operation-s1 gnal rrives supplied to terminals 10 and 11 transmitted through amplifier 12' and transformer- 14 to modulator 87- where they arefcolnbined with carrierwa've's 'from source 10 to produce signal modulated carrier waves in the outputcircuit thereof; The latter Waves are supplied through amp'lifier 38.to.transfornier 19. and terminals 16 andl'l'.

- which includes the amplifier 20 and relays 21, 22and 28, such as thoseincluded Within 7 the large broken line rectangle of Fig; 1.,

v I reircuit connected to terminals 16 .and17 j "thisnainplifier mayineludethe primary wind- 7 I p 7 p p I lease at a level 3 represented a by abscissa ,Assume that the signallevel at the zero t me ordinate has its maximum valueat the this condition and. substantially no noise 'is f 'releases'and adds noise to bringthe com j 8N for example. If this level nowrises v r v. 7 7, to .or abOVe-RQQ O', see dotted curve A I'The arrangement representejdbybloek 3Y9,

, modulated carrier-ivavesfinstead of'signalf Waves, are supplied through transformer 19'} I to theinput circuit of tube 80. These,inod-; ulated waves are detectedby device 8O and the space current thereofi accordingly viiiff riesvwith the amplitude or volume of..si g i nals as was the case in Fig. 1.' High free quency componentsof the space current pass through condenser 84 but low fr'equencyva-f riations are transmitted through transfer: mer 82 to the input Circuit of tube .2Oto control the operationof relays 21,- 22and 23, and therebythe level of noise supplied through transformer 18 to the input circuit of modulator 37. The-relays 20, 21, and 5 22 used in connection with Fig. 2 preferably have different margins otoperationl and release asindicated on the graph shown infFig. 2. This is desirable in order to,"

Represented I by I issa R21 0 and to re- {T R21+R Likewise relay '22 nay'operate at: j alevel represented at'RQQ-O; and'rele'ase g j at a'level indicated by 'R22 ,R. Relay f would then operate at the level R23#O and,

releaseat the level R23+R 1 10 5 i point. R21 O as shown by curve A. la':ys21,' 22-.and- 23 then an eperatejundera combined with. the signals *in the modulator input circuit because. resistances 30, 31 and; r 32 are all. included-in the noise -current 'cir- 1 cuit The signal level may now fall to f1 I point R21.R before any relay releasesl NVhen this point ispassed relay 21 releases; This causes noise to be combined 'with 'the, 5 signals to bring the totallevel of noise and" signalto a point, for exampl rSN If this i level of signal and 'noiserises tofBQl O; see dotted curve A relay 21 will again operate and: remove the noise. If, onthef otherkhand the level of signal "and'noise" fall'sbelow the level H22R then relay 22' binedlevel ofTsignal-andnoise, to a point" i 0 re:. ay 22"operates and cuts off the correspond; ing amount of noise. 51f: it rises to B21 0 relays 21-jand 22botlroperate; and cut. oil? I all noise. Similarly, if the ilevel r'of coin; Q 9" bined noise and signal falls to R23R, noise is added to bring the combined level again to some point SN and if the level rises above any one of the values R23O, R22O or R2l-O the Corresponding amounts of noise will be removed.

In Fig. 3 an arrangement, in many respects similar to that of Fi 1,is illustrated. However, in this instance. the input circuit of tube 20, which is coupled by transformer 19 to the terminals and 11, includes a grid leak resistance 41 shunted by a condenser 42 and the output circuit of tube 20 includes, in place of the windings of relays 21, 22 and 23, a low pass filter LPF and a resistance 43. The filter LPF is designed to pass frequencies below the audible range and to attenuate frequencies within and above the audible range.

The resistance 43 is included in the input circuit of a second tube 85 in series with a polarizing battery 86 which is normally short-circuited by a switch 87. The negative voltage produced by the drop in potential along resistance 13 may be adjusted so as to cause tube 85 to operate on a curved'pon tion of its characteristic curve. 1 Otherwise ill) the battery 86 may be included in the grid circuit by opening switch 87 and this battery thenmay have a voltage such that in conjllIlCtlOliWlth the drop in potential in resistance43 the desired voltage is supplied to the grid of tube 85. The output circuit of the tube 85 includes the primary winding of transformer 88 in series with batterv 85). The secondary winding of transformer 88 is included in a circuit comprising multiple frequency generator 33 and adjustable resi'stance 34:. Transformer 18 has its primary winding connected in parallel with the secondary wlndlng of transformer 88 and its secondary winding connected in serles With the secondary winding of transforn'ier 1%,

7 pedance whereby noise currents may be supplied from the generator 33 to the terminals 16 and 17 to be combmed with signal waves.

The secondary winding of transformer 88 f te ntial of the tube control eleetrode'in inverse proportion to the amplitude of varia- (ill tions in the space current of tube 20 supplied to its input circuit by the resistance 43. The variations in the space current of tube 20 are proportional to the variations in amplitude of the signal waves.

As the volume of signal waves supplied a) the input circuit of tube 20 increases. the

l a amplitude of the space current of tube 20' increases accordingly. The negative potential established upon the control electrode of tube 85 by the drop in potential in resistance 43 is accordingly reduced. The reduction in this negative potential on the control electrode causes a corresponding reduction in the impedance of the space path of the tube, which reduction is reflected in the impedance of the secondary winding of transformer 88, as viewed from the terminals of the primary winding of transformer 18. Hence, more of the noise currents are shunted through the winding of transformer 88 with a consequentreduction in the volume of noise supplied through transformer 18. It is thus seen that the volume of noise which is combined with signals in the output path including terminals 16 and 17, is varied continuously and inversely in accordance with the volume of the signal energy itself. 'T he absolute volume of noiseenergy may be ad justed in this arrangementas well as in those described in Figs. 1 and 2 by means of adjustable resistance 34 to obtain the proper masking of signalsby noise waves.

Theprincipal function of filter LPF is to exclude distorted signal frequencies from the noise circuit including generator 33.

The generator 33 of Figs. 1, 2, and 3 may be replaced by a plurality of generators, each arranged to generate a wave of a single frequency. In addition, in order to provide a further insurance against the unauthorized reception of signals, the frequencies of the waves consisting of the noise current, may be varied in any desired manner, as already mentioned.

Also, any additional desired number of relays corresponding to relays 21, 2-2, and'23 m'ay'be provided in order to increase the number of adjustments in the volume of noise.

The receiving circuit illustrated in Fig. at includes input terminals and 51 arranged to be associatedv with a source of carrier waves modulated by signal and noise waves or to a source of signal and noise waves, for example, an antenna, a wire line, or the like. The terminals 50 and 51 are connected to the input circuit of the detector 57 which may be any well-known form of detecting device, such as a space discharge tube arranged in the same general manner as tube 20. The output terminals of the detector 57 are connected to the primary winding of transformer 52 by a path including shunt resonant elements 53, 54 ant 55, each includ ing an inductance in series with a capacity. These resonant elements are prefe "ably tuned to the respective frequencies constituting the noise, whereby they offer low impedance shunt paths thereto for diverting Waves of these frequencies from the primary winding of transformer 52. Although only three of these elements are illustrated, it is,

l as

ofcourse, necessary-to provideas many ele-- .ments as 7 there i are different frequencies constituting the noise. 7 Additional elements may be connected in the manner-illustrated and tuned to the additional noise frequencies. The secondary ,WlIlCllIlg of transformer 52 connected to a receiving device 56 lllustrated as'a telephone head receiver.

Modulated carrier 'WL-VGS including noise and signal frequencies, when received at terminals 50 and 51, are supplied to detector f 57, .In the out-putcircuit of this detector,

there are produced, in'amannernoxv Well understood, currents including signal frequency .and, noise frequency components which, if appliedtoan ordinary receiver, Would 'be unintelligible to a listener. However, as these Waves are transmitted over the path connecting, the detector to thetrans-,

former 52, the noise frequency components are greatly attenuated by the resonantpaths 1 53, 54;, and 55,. hereas almost allot the cur- .ited, since it is not necessary to detect un-' "nals 5 The receiving circuit illustrated n Fig.

rentspf signal, frequencies, to'vvhich these elements offer high impedance, are transmitted Without substantial attenuation. Ac-

cordingly, these; waves are received, by de-v vice 56 and. produce audible sounds repre sentin the si 'nals.

noise currents,'th e detector57 may be omits modulated Waves in order to obtain thesig- 5 correspondsto thatillustratedin Fig. t

*ivith the exception. that the v res'onant felernents 53,54; and .55 areomitted, and other resonant eleinents 53', 54, and 55" aresubstitutedf The latter elements comprise anti l 'tothe transformer 52. Additional anti-resonant circuits similar to the ones indicated may beprovided as rec uirech and tuned'to additional noisefrequencies,

l pedance to Waves of the frequency to. which, theyare tuned, but low'iinpedanceto Waves of frequencies diii eringmateriallytherefrom.

' I By virtue of this fact, currents of noise fre quen ciesare greatly vimpeded in, theinput path between the detector "and v the transformer, Whereas currents of signal frequen-" cies are substantially unimpeded therein, Asa result,,.the receiver 56 is s'uppliedsubostantiallycnl'ywith signal Waves to which it'responds pro'ducinganf audible signah.

F-ig. 6illustrates a modification of theportion'of'th e circuit (ifFig/5 shown between the dotted lines 'alnd' Y 'Y. The space dischargetube 58 "has included in its nal "Waves at the terminals of circuits- 53,

tween noise and signal frequencies, "whereby 4 Should the terminals 5O and :51 be connected to an ordinary telephone lineand supplied With. signal Wavescombinedv with are more completelytransmitted. is to be v understood that'additional resonant elements, 5

[of thecircuit of Fig.5 included bet-Weenthe qu'encies. The terminals and 51{ area.

prises a magnetic bar 67 including windings '7 Lm i 68 and -ci69 upon] its opposite ends, Which reeds 71 comprising veightsi'2 and springs; -,heant1-resonant circuits offer highnnvided, with windingsconnected in a circuit example, noise and signall frequency' ccm- 51, a corresponding alternating 'inagnetie and 62 arerespectivelycoupled to the 1n 1ue-,

tancesor" th'e'elements 53, 54 and.' The Waves of noise trequency applied to thein- 7.

'put circuit oftube 58 are amplified, therein and are ted ba'ck froln' the output'circuit of, the tube to the anti-resonant circuits 53,

5i,'and 55included in the input circuit By virtue of this feeding-backot energy to my the anti-resonant circuits, theirimpedance to Waves ofnoise frequency is increased with v the result that their discrimination to the e 5 noise components-is correspondingly" increased; However, since the voltage of sig} 1 54 and 55 is 10W, little it any ener'gy" of these frequencies is transmitted tojthe input 3 'circuit'of tube58. fThis" arrangement therer I tore provides "sharper discrimination" 'be-E 1 the noise frequencies are more completely suppressed, Whereas thefslgnal frequencies similar to' those illustrated,"may 'be pro- .,q

videdin' order to suppressall o-f'the noise frequencies that maybe present in the cir cuit." v

The receiving circuit, villustrated inJ- Figi I 7,- may likewise be substituted for theportion i dotted lines X-X and ,Y Yi Thiscircuit includes a mechanically resonant systemfor discrin lnating between noise and signalfre- I ranged to receive 'si'gnaland njoise aves comb ned wh ch are suppl ed to the windings} j, of {field magnet vAn; armature 66 .co-mwindings are connected in series to" a battery 70.. "Upon the bar .67 are mounted vMined The various'reeds are tuned to mechani-' cal resonance at 'the' respective noise fre q nc s- -1 e .1

A receiving magnet Tatinbludesin its mags netic'vfield the Winding 69 -andisitselfpro1 including the j primary .Windiiig of transformer 77 and a' battery'78il fBattery 78 produces a ifioiv"ofcurrent in the 'windin'gs of;

magnet 74, whereby a constant magnetic field;

is established betivcenits poles, 1 r N 'W'hen alternating currents includi pfqr;

ponents are sup'plied to the terminals 50; and j field'is established"between-thepoles of inag net 65. This varying magnetic field establishes mechanical forces in the conductors of coil 68 by virtue of the current flowing therei reeds 71, high impedance is offered to vibrations of the frequencies to which these reeds are tuned, namely, the noise frequencies. However, materially less impedance is'offered to waves of other frequencies, i. e. signal frequencies, whereby the vibrations at these frequencies are transmitted to the coil 69. The vibration of coil 69 in the field of magnet it induces in the windings of this magnet currents including components of the signal frequencies without components of the noise frequencies. The waves represented by these currents are'then transmitted through transformer 77 to terminals 75 and 76, to which a receiving device may be connected.

The principles of this invention as described in connection with the particular embodiments illustrated are applicable to still other modifications that will readily occur to persons skilled in the art, Therefore, the scope of the invention'is to be determined only as indicated by the appended claims.

\Vhat is claimed is:

1. In a signaling system, a source of signal waves, a source of noise waves, means for combining said noise waves with said signal waves, and means independent of said first mentioned means forcontrolling the amplitude of noise waves in accordance with the amplitude of signal waves.

2. In a signaling system, a source ofsignaling waves, a source of noise waves ineluding a plurality of waves of different frequei'icy, a path traversed by both said noise and signal waves, and means including a varlable resistance controlled by a variable 1mpedanee device for varying the amplitude of noise waves in said path.

3. In a signaling system, a source of noise waves, a source of variable amplitude signal waves, means for combining waves from both said sources, said means including a variable impedance for varying the amplitude of the combined noise waves in inverse proportion to the variations in amplitudeof said signal waves.

4. In a signaling system, a transmitting station and a receiving station, a source of signal waves at said transmitting station,

1 means for transmitting said signal waves to said receiving station, a signal receiver at said receiving station, a source of noise frequency waves, said source being controlled by avariable impedance device supplied with signal waves whereby noise waves may be combined with the signal waves transmitted to said receiving station, and means at said receiving station including frequency selective elements for excluding waves of noise frequency fromsaid receiving device.

5. In a signaling system, a receiving circuit supplied with signal waves rendered unintelligible by combined noise frequency waves, a selective path designed to exclude noise frequency waves but to t 'ansmit signal frequency waves, said path including a plurality of anti-resonant circuits in series to discriminate against said noise frequency waves, a space discharge tube including an input circuit and an output circuit, said anti-resonant circuits being included in said input circuit, and a feed-back connection from said output circuit to said input circuit whereby the efliciency of said anti-resonant circuits to discriminate against said noise frequency waves is substantially increased.

6. A source of signal waves, a source of noise waves, means for controlling the amplitude of noise waves, said means including an impedance traversed by said noise waves, and means for varying said impedance in predetermined steps.

7. In asignaling system, a source of signal waves and a source of noise waves, a transmission circuit traversed by said signal waves and an auxiliary circuit traversed by said noise waves, a coupling between said circuits whereby noise waves are combined with sig nal waves, a control circuit supplied with said signal waves, means for coupling said control circuit to said auxiliary circuit, said means being connected in shuntto said first mentioned coupling, means to vary the amplitude of noise waves supplied to said transmission circuit.

8. The method of secret. signaling which comprises, combining, with signal waves, noise waves of a plurality of discrete frequencies and Varying the amplitude of noise waves in inverse proportion to variations in amplitude of signal waves. 9. The method of secret signaling which comprises combining, with signal waves, noise waves including a plurality of different frequency components and separately controlling the amplitude of the component frequencies of the noise waves by said signal waves.

10. The method of rendering signals unintelligible which comprises combining with signal waves, noise waves, and varying the amplitude of said noise waves in inverse proportion to the amplitude of said signal waves.

11. The method of secret signaling which comprises generating signal waves of varying energy content, combining noise waves with said signal waves, and varying the energy content of said noise waves whereby the combined energy ot noise and signalw'vaves s maintained substantially constant.

12. In a secret signalingsystem, a source of signal Waves, a transmission path for said U'WEIVGS, a second path coupled to said transmission path, said second] path including a s )ace' discharge device having an input cir-- cuit coupled to said source and an output circuit, a plurality of marginal'reiays in said 10 output circuit, contacts controlled by said refrequencies, and a circuitconnected-to said source ncluding a plurality of impedance 7 t ny name this 20th day of April AIDL; 1925.;

lays, a source of noise Waves of a plurality of" elements; said impedance, elernents'ibei ngf arranged to loe rendered ineffective in said oir cuit under controlof the contacts ofsa-id 1 lays.

In Witness whereof, I. hereunto subscribe I 

